NPS ARCHIVE 1968 NJUS, I. AN INVESTIGATION OF ENVIRONMENTAL FACTORS AFFECTING THE NEAR-BOTTOM CURRENTS IN THE MONTEREY SUBMARINE CANYON by Ingmar Joel Njus DUDLEY KNOX LIBRARY NAVAL POSTGRADUATE SCHOOL MONTEREY CA 93943-5101 UNITED STATES NAVAL POSTGRADUATE SCHOOL THESIS AN INVESTIGATION OF ENVIRONMENTAL FACTORS AFFECTING THE NEAR-BOTTOM CURRENTS IN THE MONTEREY SUBMARINE CANYON By Ingmar Joel Njus December 1968 TkU document ka& been approved ^ok pub tic K&- Izcuz and 4o£e; JUU dUVUbutloYi '■!-?. \ yards o 1080 2<5oo From C.& G.S. 5^03 MONTEREY BAY Soundings in Fathoms Scale - 1:50,000. . 10 21u50' Fig. 3. Bathymetry of the Head of Monterey Sub rr ar i ne C a n y c n . 17 III. MEASURING EQUIPMENT AND PROCEDURES A. THE TAUT-WIRE MOORING The location of the three moorings established during this investigation are shown in Figure 3. The taut-wire mooring system, as suggested by Dooley (1968) , was used for aach observation (Figure 4). The Navy 63 foot Hydrographic Research vessel was used in the launching and recovery of all moorings except for one recovery which was made by the Coast Guard Cutter Cape Wash. Each mooring consisted of an expendable section and a recoverable section. The expendable section consisted of a mooring line and a 500-pound (in air) anchor weight composed of five, cement-filled, five-gallon cans. The mooring line was 30 feet of 3/16 inch polypropylene line connected to the release mechanism of the recoverable section. It would be desirable to have the meter moored nearer to the bottom; however, due to the rock outcroppings, steep slopes, and unknown topographic features in the area, the longer mooring line was used to reduce the possibility of damage to the meter system. The recoverable section consisted of the release mechanism, six feet of 3/8 inch nylon line, the Savonius rotor current measuring system, nine feet of 3/8 inch nylon line, a 23 inch diameter, aluminum, buoyancy sphere, 600 feet of 3/16 inch polypropylene line (weighted with a ten-pound shot 200 feet below the surface), and a surface marker buoy. Swivels were inserted at each shackle connection to allow free rotation. A 1/4 inch plywood vane (2 feet by 3 feet) and a meter mounting frame were added to the Savonius meter system 18 Surface Float 10-pound weight 100 fathoms 3/16 inch Polypropylene line 23" diameter aluminum Buoyancy Sphere 9! 3/8" nylon line Savonius Current Meter 6' 3/8" nylon line Timed Explosive Release 30' 3/16" polypropylene line 500-pound anchor weight fttft rrr///r/r Trrmm Fig. 4. The Taut Wire Mooring 19 to relieve the strain on the system and to provide rotational stability, The stabilizer vane was balanced by an 18-inch glass float attached opposite the vane. B. THE CURRENT MEASURING SYSTEM The sensor used in this investigation was a Model 501B, in situ current speed, direction, and temperature recording system, manufactured by Hydro Products, San Diego, California. Specifications given in this section are furnished by the manufacturer. The system includes a Savonius rotor, a direction vane, and an instrument package which includes a chart recorder, battery, thermistor, and readout electronics. The underwater speed sensor is a precision balanced, high impact, polystyrene, Savonius rotor mounted on carbide to carbide, self- cleaning, bearings. It rotates at an angular rate of 83.5 revolutions at one knot. The useful range is 0.05 to 7.0 knots with an accuracy of ±3% of the reading (Franz, 1967). The rotation rate is determined from a magnetic pick-up whose output is a pulsed DC voltage which gives ten pulses per revolution of the rotor. Current direction is measured with reference to magnetic north. The direction vane is magnetically coupled to the slider of a low torque potentiometer which is referenced by a magnetic compass. By applying a fixed voltage across the potentiometer, the voltage output on the slider is proportional to its degree of travel from magnetic north. Accuracy of the direction value is ±5 degrees. 20 The temperature sensor consists of a thermistor imbedded within the aluminum housing of the electronics sphere. It is accurate to within ±3 per cent of the value over a range of zero to 40 degrees Celsius. The thermistor stability is better than one per cent error in 24 hours. Its linearity is within ±2 per cent of straight line characteristics . The electronics sphere contains the electronic circuitry, Rustrak two inch strip chart recorder, six volt nickel cadmium battery, and clock timer. Current speed read out circuitry consists of a pulse rate counter which generates a DC voltage proportional to the number of pulses received from the rotor. This DC level is recorded as current speed. Current direction readout is the amplified DC voltage from the compass slider. Water temperature is recorded on the strip chart as a measure of the DC level from the thermistor bridge. The three DC outputs are applied through a switching system to the recorder. The Rustrak recorder provides a record of temperature, speed and direction with time. The recording cycle is 7.5 minutes during which period the speed and temperature are recorded for 1.5 minutes followed by a five-minute record of speed and direction. Each input is recorded on a four-second, time sharing basis. The recorder then turns off for the remainder of the cycle. The recorder may be set for a 30-day or a 7-day record. The timer starts a new cycle on an exact 7.5-minute interval each half hour for the 30-day record, and runs continuously on 7.5-minute intervals for the 7-day record. 21 The current meter weighs 38 pounds in sea water. Its frame is 102 cm. in length and 40 cm. in width. Table I is a summary of the manufacturer's measuring system specifications. C. THE RELEASE MECHANISM A timed release, the Braincon Model 422 release mechanism, manu- factured by Braincon Corporation, Marion, Massachusetts, was used to permit the measuring system to return to the surface for recovery. After the timer counts down to the preset release time, an explosive squib is activated, which allows the release to disengage from the mooring line. The weight of the release in water is seventeen pounds. D. MOORING PROCEDURE Navigation was by radar and visual fixes on navigational aids in the Moss Landing area. Depths were determined using a Precision Depth Recorder. The surface marker was towed behind the vessel while approaching the mooring site. At the mooring site the recoverable portion of the array was lowered into the water. When the vessel was positioned at the desired location, the anchor weight was launched. Table II contains the positions, depths, and dates of the moorings. E. WIND RECORDING EQUIPMENT Wind records were obtained from the California State Colleges Marine Laboratory at Moss Landing. They have a propellor and vane type anenometer coupled to a continuous strip chart recorder. The anemometer is located on the beach directly east of the site where the current arrays were positioned. The anemometer height is approximately 22 40 feet and it is completely unobstructed from the seaward side. Wind speed and direction were recorded continuously on a Weather Measure Corporation Wind Speed and Direction Chart //C101, by means of two inked styluses. The chart advanced at the rate of one inch per hour. F. TIDE RECORDING EQUIPMENT Tidal records were obtained from a standard recording tide gauge maintained by the Postgraduate School on Wharf No. 2 in the Monterey Harbor. The heights of the tide are the same at Moss Landing as at Monterey. The time difference is about three minutes and was neglected G. WAVE RECORDING EQUIPMENT Wave data was obtained from a Mark IX Ocean Wave Recorder made by Mark Instruments, La Jolla, California. The recorder displays a continuous record received from a Snodgrass Mark IV pressure sensitive wave sensor located off Del Monte Beach. The system is maintained by the Postgraduate School. 23 TABLE I CURRENT METER SPECIFICATIONS SPEED SENSOR Angular Rate 83.5 Revolutions at one knot Useful Range 0,05 - 7.00 knots Accuracy < 3% of the reading Threshold Velocity 0.05 knot DIRECTION Accuracy i 5 degrees TEMPERATURE Range 0.00 - 40 degrees Celsius Accuracy ± 3% ol the value Thermistor Stability Less than one per cent error in 24 hours Linearity Within ± 2% of straight line characteristics i 24 co CO e OJ pei 4-1 QJ QJ C/J <-i X> m co co -h co XI CO 4-1 Pu QJ C/j O CO o 4-1 O CO 4-1 nJ 4-1 > o X j-i P- > P-, CO co o CD rH CO 0) CO QJ T3 CO 1 CO 4-1 O CO M T3 4-1 4-1 O 4-1 4J o cu o o o O a CO u c C o c o o T3 01 o vO r^ TJ O 5-i -H CO rH CO o qj o r- m co o # # • • • • •H r~- oo 00 00 r-~ o\ 4-> st vO vO Pi J 1 " ""■■*"' ' " CO H rH CO 00 00 00 i CO 00 i TJ vO vO vO vO vo vo ; QJ •-%, Tj QJ 4-1 4-1 4J • p e p- a a, 4J 4-1 4J W -H QJ QJ QJ O CJ O -*. U C/J C/J CO O O O t> QJ rH o o o o o o i 4-J CO co o co m m m U O o o CN CN o o : CO O rH rH rH rH rH o : 4-) rH O 00 vO CO r*» -d- j C/J >-' rH rH CN O rH CN 1 00 C •H 5-1 M H M O H M O h-\ S 25 IV. DATA ANALYSIS A. SAMPLING PROCEDURE AND DATA REDUCTION OF CURRENT RECORDS The Rustrak recorder provided a record of temperature, current speed and direction with time. The recording cycle of the system is 7.5 minutes. A sampling interval of 3.75 minutes (0.0625 hours) was used to convert the speed and direction records to digital form. The temperature was recorded every 7.5 minutes and the intermediate values were interpolated, which gave the equivalent of a 3.75 minute sampling interval. This interval allowed analysis of frequencies up to 0.133 cycles per minute, the "cut-off" or Nyquist frequency. B. SAMPLING PROCEDURE AND DATA REDUCTION OF WIND RECORDS The wind speed in miles per hour and the direction were recorded continuously at the rate of one inch per hour. The record was digitized at 1/16 of an inch giving a sampling interval of 3.75 minutes. Wind speed was converted to knots before performing a statistical analysis. C. SAMPLING PROCEDURE AND DATA REDUCTION OF TIDAL RECORDS The tide at Monterey is of the mixed type, in which the two high waters and the two low waters each day exhibit a diurnal inequality. The marigram for each observation period was digitized using a sampling interval of 3.75 minutes and the tidal level was referenced to Mean Lower Low Water (MLLW) . 26 D. ELEMENTARY STATISTICS All of the above data was transferred to IBM punch cards by personnel at the Postgraduate School Computer Facility. "Current," a Fortran IV computer program, (see Appendix A) written by Dooley (1968) , was modified to provide the following statistical and graphical outputs for each time series: 1. Hourly means 2. Daily means, medians, modes, and frequency distributions 3. Time series means, medians, modes, variances, standard deviations, and frequency distributions 4. Histograms. These elementary statistics were computed using standard techniques with the exception of direction means which were computed by the method of Webster (1964) . For analyzing wind records additional steps were inserted in the current program to change wind speed from miles per hour to knots and to alter the conventional wind direction by 180 degrees for easier comparison with direction of the current flow. Separate programs were written to provide a graphical display of each time series (i.e. temperature, wind speed and direction, and tidal fluctuations) . The above programs were all run on the Postgraduate School IBM 360 computer. Summaries of the wind and current statistics are listed in Tables III-VI. E. POWER SPECTRA ANALYSIS Spectral analysis was applied to the individual time series to divide the variance of the time series into discrete bands. The power spectrum provides a measure of the density of the variance as a function of frequency. Power spectral computations were made on 27 the CDC 3600 computer at Fleet Numerical Weather Central, Monterey, using computer program GHOST, written at Scripps Institution of Oceanography, La Jolla, California. This program also provided coherence between selected time series. Frequencies between 0.133 cycles per minute and 0.375 cycles per day were analyzed. Normalized power spectra are plotted in Figures 22-27. 28 w 1 ———j 1 CM CM CM S3 '"N 1 •"-s /— \ , H O H H EH , Z M H o | H H EH e H H H o H H W Eh o ^— ' ! o o 0 ■^^ o 0 o -*s 0 o . P4 CJ 3 W ^> 3 m O CM m CTi o CT> CTi CO CT> C_> M H vD rH rH CM rH CO CO • H 4-1 ^ ±J 4-J -U a: J-> ■u ; z O • : 1 • ,M ^ ^ ^ w ^J ^ w w "1 CT. 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Power Spectra of Current Direction (normalized energy density vs. period in hours) 60 t.o-r o.S o.o Cruise I (data net available) X3 Cruise II XX J XO c •H to C t Q >> W) U o c w /.Ot Cruise III o.S ■ o.o x.xs x.o Fig. 24. Power Spectra of Current Speed (normalized energy density vs. period in hours) 61 Cruise I o.s • l.o Cruise II %.o n Cruise III l.o Fig. 25. Power Spectra of Water Temperature (normalized energy density vs. period in hours) 62 I.o-r Cruise I OS o.o l.o Cruise II 1.1s x.o -p •H C >» -1-5 •H CO C I.O t a Cruise III o.s- o.o 3T- iy /*. i-r l.*S 2.0 Fig. 26. Power Spectra of Wind Direction (normalized energy density vs. period in hours) 63 os- o.o Cruise I 2.S X.O X) Cruise II Cruise III X.o Fig. 27. Power Spectra of Wind Speed (normalized energy density vs. period in hours) 64 VI. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS A. SUMMARY Continuous measurements of water temperature., current speed, and current direction were taken for three 7-day periods in the head of Monterey Submarine Canyon. The arrays were positioned along the canyon axis at 80, 90, and 110 fathoms, with the measuring system about 40 feet above the bottom. Current flow was influenced predominantly by the canyon topography. The directional distribution was bimodal in the up and down-canyon directions. Current speeds in excess of one knot were common throughout the records. The average speed was 0.58 knot (standard deviation: 0.28 knot) which is considerably higher than previously recorded values of current speeds in this or any other canyon. The range of the tide appears to have a direct influence on the maximum speed attained in each cycle. Water temperature, current speed, and current direction all exhibited periodic fluctuations corresponding to the tidal cycle. Results of this investigation substantiate the findings of Gatje and Pizinger (1965) in that the reversal of the near-bottom current direction coincides closely with the times of high and low tide. And further, that the direction of flow is contrary to what would be expected, i.e., up-canyon on the falling tide and down-canyon on the rising tide. The down-canyon flow was generally of a greater magnitude and sustained for a longer time than was the up-canyon flow. 65 The temperature distribution shows a wider range of warmer temperatures for the last two cruises than for the first. Warmer water was associated with the seaward (down-canyon) current direction, while a reversal to up-canyon flow always resulted in a marked decrease in water temperature. Spectral analysis failed to reveal any significant correlation between wind speed or direction and the current flow. There appears to be a relationship between wave period and the maximum current speeds attained, however this was not investigated in detail and should be the object of future research. B. CONCLUSIONS The primary environmental factor influencing the near-bottom currents in Monterey Submarine Canyon is the semidiurnal tidal fluctuations. The correlation between current direction and tidal phase is very good as long as the semidiurnal tidal components are nearly equal. When there is a mixed tide, the relationship becomes vague. However, as the tide returns to having equal semidiurnal variations, the relationship with current direction again becomes readily apparent. The near-bottom currents may be explained as a seaward return flow of inshore water movement due to tide and wave/swell activity. The periodic fluctuations of water temperature and current speed are probably directly related to the tidal cycle and are manifestations of the reversal of the current direction with change in the tidal phase. 66 C. RECOMMENDATIONS Future current studies should be of a longer duration to allow greater resolution of the power spectrum calculations. To better define the nature and extent of the near-bottom currents multiple arrays should be used in various configurations (e.g., two or more meters arranged vertically on a single moor, simultaneous moorings at different depths along the canyon axis, or one array in the canyon with another measuring water motion on the continental shelf nearby ) . Present data should also be re-evaluated after filtering out tidal components to determine if there are other significant periods of oscillation, which may be obscured by the tidal influence. 67 BIBLIOGRAPHY Dooley, J.J., "An Investigation of Near-Bottom Currents in the Monterey Submarine Canyone," Unpublished Master's Thesis, Naval Postgraduate School, Monterey, California, June, 1968. Franz, F. , "Model 501 insitu Current Speed, Current Direction, and Temperature Recording System, Operation and Maintenance Instructions," Hydro Products, San Diego, California, June, 1967. Gatje, P.H., and Pizinger, D.D., "Bottom Current Measurements in the Head of Monterey Submarine Canyon , " Unpublished Master's Thesis, Naval Postgraduate School, Monterey, California, 1965. Martin, Bruce D. , and Emery, K.O., "Geology of Monterey Canyon, California," The American Association of Petroleum Geologists Bulletin, Vol. 51, No. 11, pp. 2281-2304, November, 1967. Shepard, F.P., Curray, J.R., Inman, D.L., Murray, E.A., Winterer, E.L., and Dill, R.F., "Submarine Geology by Diving Saucer," Science, Vol. 145, pp. 1042-1045, September, 1964. Shepard, F.P., and Emery, K.O., "Submarine Topography off the California Coast: Canyons...," Geological Society of America Special Papers No. 31, pp. 72-78, 1941. Shepard, F.P., Revelle, and Dietz, R.S., "Ocean Bottom Currents off California Coast," Science, Vol. 89, No. 2317, pp. 488- 489, May, 1939. Skogsberg, Tage, "Hydrography of Monterey Bay, California. Thermal Conditions, 1929-1933," Transactions of the American Philosophical Society, Vol. XXIX, December, 1936. Stetson, H.C., "Current Measurements in the Georges Bank Canyons," Transactions of the American Geophysical Union, Vol. 18, pp. 216-219, July, 1937. Webster, F. , "Processing Moored Current Meter Data," lecanical Report No. 64-55, unpublished manuscript, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, December, 1964. 68 APPENDIX A oo »- 3 ix. a LLC^X h- X<< 3 •> •— a.OO — i-oo^a: O ZO<_J O UJUJ^OZl-J ^ O x iki a < ►- OOQ. * C — a;Z ocr O luq •- GG ZC O »-lu — u_t— •< O <>- h-OO _J f*t OOO • OOOUJUJ*! «- »3 > uai'-'UZ oo c~5 — uj «-"a:ctzO •> XQ CO O OO • UJ (—<•—■ «- •— >»— CD QL OO (/) OO t— • t— »O0 C ►- UJ >oujoouj -* a.x k 3 x OfLU UJt-i o •• oo O z •-< -« h_ X O K Zooh- luoo m ►ae cvji—ta-a. uj — uj < z uj oo — ZQ i-.ww z «t— « z cc a a x -cc uj uj a. ••zo'i. 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